1. Field of the Invention
The present invention relates generally to flame retardant optical fiber coating compositions, and, more particularly, to flame retardant fiber optic coating compositions that are durable when cured. The invention also relates to an optical fiber coated flame retardant coating composition, and to methods of making such optical fiber. The invention further relates to tight-buffer flame retardant fiber optic coating compositions and to methods of making such optical fiber.
2. Description of Related Art
Optical glass fibers are frequently coated with two or more superposed radiation-curable coatings which together form a primary coating immediately after the glass fiber is produced by drawing in a furnace. The coating which directly contacts the optical glass fiber is called the “inner primary coating” and an overlaying coating is called the “outer primary coating.” In older references, the inner primary coating was often called simply the “primary coating” and the outer primary coating was called a “secondary coating,” but for reasons of clarity, that terminology has been abandoned by the industry in recent years. Inner primary coatings are softer than outer primary coatings.
Single-layered coatings (“single coatings”) can also be used to coat optical fibers. Single coatings generally have properties (e.g., hardness) which are intermediate to the properties of the softer inner primary and harder outer primary coatings.
The relatively soft inner primary coating provides resistance to microbending which results in attenuation of the signal transmission capability of the coated optical fiber and is, therefore, undesirable. The harder outer primary coating provides resistance to handling forces such as those encountered when the coated fiber is ribboned and/or cabled.
Optical fiber coating compositions, whether they are inner primary coatings, outer primary coatings, or single coatings, generally comprise, before cure, a polyethylenically-unsaturated monomer or oligomer dissolved or dispersed in a liquid ethylenically-unsaturated medium and a photoinitiator. The coating composition is typically applied to the optical fiber in liquid form and then exposed to actinic radiation to effect cure.
Optical fiber comprising a waveguide, an inner primary coating and an outer primary (or secondary) coating typically has a diameter of approximately 250 μm. The inner primary coating typically has an applied thickness of 20-40 μm and the outer primary coating typically has an applied thickness of about 20-40 μm.
For the purpose of multi-channel transmission, optical fiber assemblies containing a plurality of coated optical fibers have been used. Examples of optical fiber assemblies include ribbon assemblies and cables. A typical ribbon assembly is made by bonding together a plurality of parallel oriented, individually coated optical fibers with a matrix material. The matrix material has the function of holding the individual optical fibers in alignment and protecting the fibers during handling and installation. Often, the fibers are arranged in “tape-like” ribbon structures, having a generally flat, strand-like structure containing generally from about 2 to 24 fibers. Depending upon the application, a plurality of ribbon assemblies can be combined into a cable which has from several up to about one thousand individually coated optical fibers. An example of a ribbon assembly is described in published European patent application No. 194891. A plurality of ribbon assemblies may be combined together in a cable, as disclosed, for example, in U.S. Pat. No. 4,906,067.
The term “ribbon assembly” includes not only the tape-like ribbon assembly described above, but optical fiber bundles as well. Optical fiber bundles can be, for example, a substantially circular array having at least one central fiber surrounded by a plurality of other optical fibers. Alternatively, the bundle may have other cross-sectional shapes such as square, trapezoid, and the like.
Coated optical fibers (or waveguides) whether glass, or, as has come into use more recently, plastic, for use in optical fiber assemblies are usually colored to facilitate identification of the individual coated optical fibers. Typically, optical fibers are coated with an outer colored layer, called an ink coating, or alternatively a colorant is added to the outer primary coating to impart the desired color.
The ink layer, if applied, typically has an applied thickness of about 4-8 μm. The optical fiber, coated with inner primary coating, outer primary coating, and ink layer typically has a diameter of about 260 μm.
Typically, the matrix material of a fiber optic ribbon assembly or cable is separated from the individual coated fibers in order to facilitate splicing two cables, or the connection of a fiber to an input or output. It is highly desirable that the matrix material can be removed from the coated fiber with little or no effect on the outer primary coating or colored ink coating of the fiber. Good removability of the matrix material not only preserves the readily visual identification of the color coded fiber, it also avoids harming the waveguide during the removal process.
It is well known in the art that optical fiber coated with well-known inner primary, outer primary and ink or colored coatings have a relatively small diameter that makes such fiber difficult to work with and not entirely satisfactory for handling purposes. It is known to bundle optical fiber in loose buffer tubes. Such tubes include optical fiber surrounded by a gel-type buffer layer which is surrounded by the tube material. In order to improve handleability, and to add to the protection of the optical fiber, it is known to “upjacket” the fiber with a tight buffer coating. Upjacketing of the optical fiber is typically carried out to increase the diameter of fiber of from about 250 μm to a diameter of from about 650 μm to 900 μm. Upjacketing is desirable for applications such as local area networks, in-home applications and in commercial establishments. Upjacketed fiber can be bundled without the need for additional gel filling or buffering in loose buffer tubes known in the art.
Because the optical adhesive and durability properties of the tight-buffer coating are not as rigid as those properties are for the inner primary, outer primary and ink compositions typically used to make optical fiber, thermoplastic materials such as polyvinyl chloride have been used heretofore as the tight-buffer coating. However, thermoplastic materials, such as polyvinyl chloride-based tight-buffer coatings are undesirable, particularly as the demand for tight-buffer coated optical fiber rises.
Equipment for applying thermoplastic buffer based coatings is expensive, thermoplastic materials are not suitable for short runs, and it is difficult to apply such coatings. Other drawbacks of thermoplastic coatings are that they must be heated during application, they must be extruded through relatively small dies, e.g., on the order of 250 μm to 900 μm, they must be cooled which can result in undesired stresses in the optical fiber and they are not adapted to be applied at the high line speeds at which optical fiber is made. In addition, such coatings are not readily strippable from the optical fiber. Further, polyvinyl chloride-based coatings tend to become opaque when flame retardants are added to them. The opaque coating makes it difficult or impossible to identify individual color-coded fiber and thus defeats the important purpose of providing colored fiber for differentiating the different fibers in a ribbon, bundle or cable.
Recently, the art has attempted to provide a UV light-curable tight-buffer coating. For example, U.S. Pat. No. 6,208,790 B1 describes such a coating, but this patent does not describe flame-retardant tight-buffer coatings, and it does not describe UV light-curable coatings which are flame retardant.
It would be advantageous in the art to provide a flame retardant tight-buffer coating composition suitable for upjacketing optical fiber that is curable by exposure to actinic, i.e., ultraviolet, radiation as well as such a coating that can be used on existing machinery and in existing processes well known to producers of optical fiber. Such machinery includes but is not limited to the machinery for applying ink to coated fiber and to ribbon-making machinery. Additionally, it would be desirable if the flame retardant tight-buffer coating is optically transparent and durable.
Thus, there remains a need for a UV-curable flame retardant material that is optically transparent and that is durable, and, in one particular aspect, for a tight-buffer material that is optically transparent and that is durable. The present invention provides a composition that has these and, optionally, other attributes as well.